This invention relates to gas-filled bushings for two-pressure circuit breakers, and more specifically relates to a novel arrangement for increasing the current rating of a given gas-filled bushing and for ensuring the dryness of the gas in a gas-filled bushing when used in a two-pressure system.
Gas-filled high voltage bushings are well known in the electrical power art. A typical high-voltage gas-filled bushing is shown in U.S. Pat. No. 3,566,001 in the name of McCloud, issued Feb. 23, 1971 and entitled GAS FILLED BUSHING WITH SPRING BIAS CLAMPING AND INTERNAL FLEXIBLE SHUNT. Bushings of this type are used for high voltage two-pressure gas blast circuit breakers of the type shown in copending application Ser. No. 398,871, filed Sept. 19, 1973, now U.S. Pat. No. 3,909,571, in the name of Aumayer, entitled CONTACT STRUCTURE FOR HIGH VOLTAGE GAS BLAST CIRCUIT INTERRUPTER, and assigned to the assignee of the present invention. A high dielectric gas such as sulfur hexafluoride or other gases mixed with sulfur hexafluoride fills the bushing and communicates with the interior of the low-pressure main housing of the circuit breaker which receives the bushing. The gas is stationary within the bushing and circuit breaker housing and is at a low pressure of, for example, 3 atmospheres. The gas is supplied from a high-pressure container which will be at a pressure of greater than about fifteen atmospheres. The bottom of the conductive studs of the bushings are then electrically connected to the contacts of interrupter structures which contain blast valves to allow high pressure gas to blast through the contacts and into the low pressure housing when the contacts are operated.
The contacts of the interrupters generate heat at their contacting surfaces and this heat is transmitted to regions external of the enclosed circuit breaker by conduction along the central bushing conductor. Heat is also generated due to current flow in the central bushing conductor itself which must also be conducted to the top terminal of the bushing through the bushing conductor and its shunts and the like.
The gas and porcelain enclosure surrounding the central conductor of the bushing form a poor heat transfer medium so that auxiliary means are needed to remove heat from the bushing conductor, particularly if currents are applied to the bushing beyond those for which the bushing was originally designed. That is to say, if a particular circuit breaker is to be redesigned for a higher current rating, one of the major elements of the redesign is the modification of the bushing for higher current ratings. The present invention allows the use of existing bushings at increased current ratings.
The prior art bushing arrangement complicates the installation of the breaker into service since time-consuming evacuation techniques are necessary to ensure a dry interior before dry SF.sub.6 gas is loaded into the circuit breaker. Moreover, during service and under certain temperature conditions, moisture absorbed in the gas may condense on the upper regions of the insulator bushing. More specifically, and in the prior art arrangement, during installation of a breaker, the high-pressure and low-pressure systems are evacuated to remove the air and moisture. During this time, the blast valve of the interrupters is held open to permit thorough evacuation of the moisture from the high and low-pressure systems. During service, the gas in the bushings, which is normally stagnant, communicates with the low-pressure circuit breaker system through holes and filters at the bottom of the bushing.
When the breaker is operated to affect arc extinction, the gas from the high-pressure system is exhausted through the interrupter into the low-pressure system, thus the high and low-pressure systems tend to equalize in pressure. A compressor is provided which switches on at a given pressure level in the high or low-pressure systems to recompress the gas from the low-pressure system into the high-pressure system and to restore the system to its desired pressure differentials. During this operation, gas will flow through in-line filters and the moisture in the gas will be absorbed. This then causes a reduction in the moisture content in the high-pressure gas. Upon the next operation, this dried gas is then exhausted into the low-pressure system and so on.
If the temperature in the system including the low-pressure housing and the circuit breaker bushing were the same everywhere, then the partial pressure of the water in the low-pressure gas would also be the same. The partial pressure of water equalized by migration of moisture through the various openings permits communication between the bushing interior and the low-pressure tank interior. As a practical matter, however, the temperature throughout the system is not constant and generally the temperature at the top of the bushing will be lower than the temperature of the low-pressure tank due to the localized generation of heat by the through current, the wind factor and the like. Thus, the moisture per given volume of gas in the bushing can be higher than that in the tank. This high moisture content may then condense out on the interior surfaces of the bushing at its upper regions, thereby seriously lowering the dielectric withstand strength of the bushing.
As a further disadvantage in the present system, when the breaker is not called upon to operate frequently, the compressor will turn on only infrequently so that the moisture absorption effectiveness of the filters is minimized. To partially offset this, bags of desiccant material are disposed within the low-pressure tanks.